This invention relates to a process for the fractionation of a gaseous mixture under pressure, this mixture containing generally methane, ethane, propane, inert gases such as nitrogen, higher hydrocarbons (C.sub.4 +), hydrogen sulfide and carbon dioxide.
In heavy natural gases having a high content of sour gas, the C.sub.4+ hydrocarbons must generally be separated before removing the sour gas from the natural gas by physical scrubbing steps since, on the one hand, further processing of the sour gases in a Claus plant is subject to a low tolerance of C.sub.4+ components, and, on the other hand, it is desirable to obtain a high yield of C.sub.4+ which can be utilized either as fuel or as a starting material for chemical syntheses. The pre-separation of the C.sub.4 + fraction is generally accomplished by fractional condensation and/or rectification.
DOS No. 2,828,498 discloses a process for natural gas fractionation by rectification and methanol scrubbing. This process is conducted in such a way that, in the rectification step, not only a large portion of the higher hydrocarbons, but also hydrogen sulfide and, in certain cases, a portion of the carbon dioxide are obtained as rectification bottoms. The methanol scrubbing step is employed to remove carbon dioxide and, in some cases, small amounts of hydrogen sulfide and heavy hydrocarbons from the rectification head product. This process however, is unsuitable for the processing of natural gases having a very high content of sour gases, e.g., having at least 1 mole percent, especially at least 5 mole percent of sour gases.